BaCuTP2 (T = Al, Ga, In): a semiconducting black sheep in the ThCr2Si2 intermetallic family

The ThCr2Si2 structure type has been well explored for decades with diverse magnetic, superconducting, and heavy-fermion behavior. For transition metal-containing ThCr2Si2-type compounds, a metallic band structure and properties are typical. In this work, a rare example of semiconducting BaCuTP2 (T = Al, Ga, In) materials is reported. BaCuTP2 materials retain the tetragonal I4/mmm ThCr2Si2-type crystal structure with a large c/a ratio of similar to 3.3, where Cu and T metals jointly occupy the Cr-site. Edge-sharing (Cu/T)P-4 tetrahedra form [CuTP2](2-) layers stacked along the crystallographic [001] direction, with the Ba2+ cations located in the interlayer spaces. Solid state NMR revealed partial short-range ordering in the Cu/T sublattice. The composition of the produced phases is electron balanced, Ba2+Cu1+T3+(P3-)(2). High values of Seebeck coefficients were experimentally observed due to the high valley degeneracy in the band structure. Heat capacity and structural studies show that Ba exhibits anisotropic 'rattling-like' behavior along the [001] direction in Ga- and In-containing compounds. A combination of Ba rattling, short range Cu/T ordering, and a recently discovered coupling between acoustic and optical phonons for ThCr2Si2-type phosphides, resulted in ultralow thermal conductivity (<0.50 W m(-1) K-1) for the title compounds.